My little engine (continued)

[roy entwistleParticipant @royentwistle24699]

Garry I would have finished the cylinder before I made the piston I would also have mounted the cylinder on the bottom end cover as per your photo on the 20th Then made the piston and piston rod before I drilled the cross head for the rod

Just my idea

Roy

[GarryCParticipant @garryc]

Hi Ian

Yes the Cylinder is also cast iron, the rings are just a few pounds to order. I did this morning order a little cheap (£6) tool for fitting the piston rings and I can see a little compressor tool as well for getting the Piston and Rings into the Cylinder. Sounds like I may not need that as well so I'll not order one of those until I see how it looks.. Thanks wish I had seen your post earlier.

Hi Jason

Its good to see that I had approx the right ideas this time, thanks I'll be following your steps again. I realised after I had posted that I hadn't asked about the depth of the grooves – and wondered if I needed to go a little deeper with the groove than the 1/8" on the drawing – 5 thou it is then, really useful / important to know that. I'll remember your last comment about getting the thread a firm fit as well, that's a good tip for use elsewhere. I'll likely stay with the screwdriver slot as I can't see how to tighten the piston on the rod when its in the cylinder, although I guess the rod could be screwed into the Crosshead after being attached to the piston maybe..

Hi Roy

Thanks, I'm only guessing – that you have a method of measuring from the fitted piston rod directly onto the Crosshead i.e.. with the Cylinder, Piston Rod and Piston in place – I used a 'spotting through bar' through the Bottom Cylinder cover (no Cylinder fitted) but it would have been more accurate with all the parts in place I'm sure..

Cheers

Garry

Edited By Garry_C on 23/07/2015 17:25:01

[roy entwistleParticipant @royentwistle24699]

Garry Until youv'e got your cylinder your piston and both covers and gaskets' if used I can't see how you can establish the length of the piston rod I wouldn't rely solely on measurements Once youv'e got every thing lined up remove the cylinder and work from there

Others may offer other advice but thats my 2 pennyworth Roy

[GarryCParticipant @garryc]

Hi Roy, thanks its certainly something to think about for future projects..

Cheers

Garry

[GarryCParticipant @garryc]

Again sorry I should have asked this above as well –

So I've been looking on the internet for guidelines on what width to cut the piston ring grooves, I presume they need to be slightly oversize to allow for them to expand with wear, also how much undersize for the piston diameter. Much of the info I have found in relation to (small scale) steam seems to concern rubber or similar 'o' rings on the piston, the only real references I've found suggests 2 or 3 thou oversize for the width of the piston ring grooves and 10 to 12 thou undersize for the piston diameter, which sounds a lot to me but then I really don't know. [I know from the above of course to go 5 thou deeper with the grooves.]

So simply put, the Piston and Rings are both cast iron how much wider should I cut the grooves and how much undersize for the piston diameter in relation to the cylinder bore – thanks for any help.

I can see many references to various noises being heard from the working engine when these things are not right – to be expected of course…

I've started a little notebook to keep all this info in for future so I don't have to keep asking etc..

Cheers

Garry

Edited By Garry_C on 24/07/2015 08:58:36

[GarryCParticipant @garryc]

and there's me thinking it couldn't get any more interesting – just in case anyone wants to comment –

Roy set me thinking and I've checked to see how the measurements are looking so far. I can see that at the lowest point of the piston rod stroke the shoulder on the piston ( this will be level with the bottom of the piston) gets as low as 1 thou above the bottom cylinder cover surface (I've measured lots of times and as far as I can tell never below this), so the piston will get this close. How does that sound? Do I need to skim the bottom of the piston (when attached to the piston rod) to increase the gap? I'll soon find out when I have it made of course but was just wondering what would be considered an appropriate gap between piston and the ends of the cylinder covers at BDC and TDC… Thanks if anyone has any views..

Cheers

Garry

sorry I've tried to add this to my last post to tidy things up but can't edit – too much time passed between the two posts I think..

Edited By Garry_C on 24/07/2015 09:52:52

[MichaelRParticipant @michaelr]

The main thing to note is that the piston does not mask the steam inlets at TDC & BDC, a clearance of no more than 1/32" will be OK.

Mike.

[Anonymous]

On my hit 'n' miss engine, for which I made my own piston rings, I cut the grooves first. Then, after making and heat treating the rings I rubbed them on wet 'n' dry on a surface plate until they were a loose fit in the grooves. On the traction engines, where I bought Clupet piston rings, I widened the groove a thou at a time until the rings had a slight angular movement. I've just measured the Clupet rings with a micrometer and the corresponding piston grooves with slip gauges; it looks like the clearance is about 2 to 3 thou. As for radial clearance I can't remember, but probably about 10 thou.

For piston to cover clearance, in theory as small as possible consistent with not covering the steam inlet ports. In practice a small gap is helpful for several reasons. When operating everything gets hot at different rates, so what might be a clearance when cold might not be at operating temperatures. A small steam volume at the end of each stroke can help to cushion the piston as it deccelerates and raises the pressure in the cylinder, which may help with initial steam flow. Finally the piston hitting either end cover is the sort of thing that can ruin ones day.

Andrew

[GarryCParticipant @garryc]

Thanks Mike, that's very useful, I can see about increasing the gap a little then when the time comes and check the inlets as you say..

"piston rod stroke the shoulder on the piston" – should read 'piston rod stroke the shoulder on the piston rod' in my last post of course.

Cheers

Garry

[GarryCParticipant @garryc]

Thanks Andrew, that will be really useful as well. I've just had a quick look at the cylinder, I've haven't machined it all yet, the steam ports in the bore look to be about 3/16" wide and at the moment its a further 3/16" to the end of the Cylinder – I'm glad now I've thought to get my head around all this before starting the Piston..

Cheers

Garry

Edited By Garry_C on 24/07/2015 10:54:18

[roy entwistleParticipant @royentwistle24699]

Garry Skimming the piston will not increase the gap thinning the end cover will If you assemble the finished cylinder with both end covers the distance between the inside of the end covers minus the thickness of the piston divided by 2 will give you clearance in the cylinder ( if that makes sense )

Roy

[JasonBModerator @jasonb]

I usually leave about 1/32" between end cover and piston, I'll take a look at teh No1 drawings a bit later and come back to you. If you do need to adjust things then the bottom cover spigot can be reduced

For iron rings in an iron or ali piston 1 or 2 thou on width and 10 thou on depth. Note the 0.005 I mentioned earlier was to allow the skimming of the piston OD to bring the groove depth back down to finished size.

As Has been mentioned above I would make the cylinder first then if you do end up with an oversize bore then piston can be sized to suit

[GarryCParticipant @garryc]

Wow I'm on a steep learning curve this morning thats for sure..!

I just hadn't been thinking this through and only just I think starting to see the relationships between the Piston Rod and Piston length, and the distance from the steam inlets in the bore to the end of the cylinder, and the cylinder length covers and gaskets – thats just for starters.. I've been too focused on little bits of the drawings at a time and not enough time spent looking at the overall picture as well, perhaps relying a bit too much as I think Roy was saying on written 'dimensions' – guilty of not enough checking on the actual engine as well..

I'm glad when I got up this morning I headed for the computer and not the workshop.

Roy, thanks I'm only just beginning to understand what you've been saying but it is going in now and Jason if you can take a quick look at the drawings sometime in case you can comment further that would be brilliant. I'll leave the piston for now then and start thinking about doing the Cylinder next instead, but I'll leave it all today or for a day or so until I'm a bit clearer on things, and my head stops hurting…

This is all knowledge learned that can be applied in some forms to future projects I'm sure. It hasn't stuck from my experience with Victoria but that was far simpler..

Obviously I'd have no chance whatsoever if I weren't on here..! Sorry to be so clueless but my eyes are now finally beginning to open and its making a lot more sense..

Thanks again everyone.

Garry

[roy entwistleParticipant @royentwistle24699]

Garry Don't worry tooo much about it We've all been there An old teacher used to say experience is built on mountains of scrap and the man who never made a mistake never made anything

Roy

[JasonBModerator @jasonb]

Looks like me and Mr Stuart Agree.

O/A length of cylinder casting is 2.875"

Into this we need to get 2x 0.031" spigots on cylinder covers, 0.750" piston and 2.000" of piston movement which totals 2.812". I have not included for gaskets as I use a liquid gasket which does not add to the joint width like paper gaskets do.

So that leaves us 0.62" space which if split evenly top and bottom of the piston means 0.031" or 1/32" clearance each end

Just got to go back out to work but there is a way to recover things without having to make a new piston rod as your 0.001" is a bit too tight.

[Anonymous]

Posted by roy entwistle on 24/07/2015 11:07:37:

Skimming the piston will not increase the gap thinning the end cover will………

That doesn't make any sense to me? Let's suppose the piston is at BDC and there is a gap of 1/32" between the lower face of the piston and the end cover. If we now skim 1/32" off the end cover the gap becomes 1/16". So far so good. But if instead we skim 1/32" off the bottom of the piston surely the gap also becomes 1/16"? If we now go round another complete revolution the piston ends up back where it was; and the gap will still be 1/16".

Andrew

[GarryCParticipant @garryc]

Agree with that saying Roy, but if I weren't on here I'd be getting a call from the Tour de France organisers with my mountain… It is a very nice feeling though when gaining some more understanding..

Jason my walls are getting covered in your postings, this one is a little gem to see the workings like that and will get lots of use in the future… I'll be very interested to hear what you suggest I do of course…

I've just stumbled on this site when looking up liquid gasket for steam engines – looks like it may be useful for lots of things..

**LINK**

Cheers

Garry

[roy entwistleParticipant @royentwistle24699]

Andrew If you skim 1/16th off the bottom of thr piston the piston will screw down the piston rod by 1/16th your back where you started really you need a longer piston rod

Roy

[JasonBModerator @jasonb]

Oh no you don't

You say that the shoulder on the piston rod is 0.001" above the spigot on the cylinder bottom cover so all we need to do is modify the piston so the majority is another 0.030" above the cover. To be safe its also worth adding a bit of clearance where the piston touches the rod shoulder.

This shows a 1/2" dia boss on the bottom of the piston that is 0.030" long.

And to clear that boss a 9/16" dia recess in the top of the cylinder cover that is also 0.030" deep

Looking from the side you can now see that there is 1/32" clearance between cylinder cover and piston

Same view but in section, the recess has come out a bit dark as the program is adding shadow but you should get the idea

I'll do a dimensioned drawing later but Le Tour becons

Edited By JasonB on 24/07/2015 16:28:30

[roy entwistleParticipant @royentwistle24699]

Jason All due respects but I think you'l find that doing it your way will increase the unswept volume at the other end of the stroke Either the cylinder wants lowering or the piston rod needs lengthening

Draw the complete cylinder , piston and crank

Roy

Edited By roy entwistle on 24/07/2015 19:22:09

Edited By roy entwistle on 24/07/2015 19:22:46

[GarryCParticipant @garryc]

Great drawings Jason thanks very much. I can see you what you mean and it looks like a great solution. The only thing I'm not sure on now (promise thats all) is the piston travel and if at BDC & TDC does the piston travel past the inlet ports in the bore or does it never pass or cover them or maybe just partially? They are already as cast about 3/16" wide inside at the bore ends.

[JasonBModerator @jasonb]

Roy as drawn the top of the piston is exactly the right distance from the crosshead pivot as per the Stuart drawing so can't see how it would affect the unswept volume.

The 0.030" boss is in effect extending the piston rod by the amount it is too short, the piston is still 3/4" thick excluding the boss as plans. Or look at it anotherway the piston has been unscrewed from the rod shoulder by 0.030" and a washer in teh form of teh boss added.

J

[JasonBModerator @jasonb]

For those that like looking at pictures here are a few more. I don't see the point in drawing the crank and crosshead but will assume the crank is to drawing and has a 2" throw. Garry has previously stated that teh shoulder on teh piston rod stands 0.001" above the top of the lower cylinder cover so here it is with my modified piston screwed onto the end.

No if I alter the position of the shoulder from 0.001" to 2.001" above the lower cover this is what we get

Looks spot on for equal gap at each end of the stroke to me. Ideally it would be nice if Garry could confirm the height of the shoulder at TDC then it would be possible to make allowances if the stroke is not quite 2" dead.

Garry regarding your other point about where the passages exit teh side of teh cylinder, it will depend on where they are cast to some extent, try to take metal off each end of teh cylinder to leave an equal amount of wall each end, I have shown 1/16". As you can see this covers the port at TDC and BDC so what is often done is to mill out a small amount of cylinder wall so the steam can get right to the end, something like this

J

Edited By JasonB on 24/07/2015 20:40:04

[Anonymous]

Posted by roy entwistle on 24/07/2015 15:54:14:

Andrew If you skim 1/16th off the bottom of thr piston the piston will screw down the piston rod by 1/16th your back where you started really you need a longer piston rod

Roy

Good point!

I'm afraid I was thinking of a piston that fitted on a plain shank with a shoulder where skimming the face of the piston, leaving a boss, would increase the gap. Pretty much as drawn by JasonB.

Andrew

[GarryCParticipant @garryc]

Jason that is absolutely superb.

[the height to the piston rod shoulder from the surface of the bottom cylinder spigot at TDC is 51.25mm.]

I have learnt so much today and its been brilliant, thank you very much indeed.

I've taken up far too much of peoples time – I can get stuck in and sort it out now…

Thanks to everyone else as well..

I'll post progress of course…

Garry

Edited By Garry_C on 24/07/2015 20:59:18

Edited By Garry_C on 24/07/2015 21:02:51

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